1. Field of the Invention
This invention relates to the remediation of contaminated groundwater, and in particular, relates to a remediation method utilizing a microemulsion of an innocuous oil.
2. Description of the Related Art
There are numerous techniques employed for the remediation of contaminated groundwater in aquifers. The mechanisms for cleanup may be physical, chemical or biological. A typical physical remediation method for groundwater contaminated with volatile solvents includes recovery of the contaminated water using a series of wells followed by above-ground treatment by air stripping and/or activated carbon adsorption.
The most common approach for enhancing the anaerobic conversion of organic and inorganic contaminants in the subsurface involves continuously flushing a soluble readily biodegradable substrate such as lactate or molasses through the contaminated zone. There is, however, significant capital expense associated with the installation of the required tanks, pumps, mixers, injection and pumping wells and process controls required to continuously feed a soluble easily degradable substrate. Operation and maintenance costs can be high because of the frequent clogging of injection wells and the labor required for extensive monitoring and process control.
Treatment of contaminated groundwater in situ is often a less expensive approach for groundwater remediation. In situ treatment technologies generally rely on the natural migration of contaminated groundwater to the treatment zone where the transformation can occur via either chemical or biological mechanisms. Most previous in situ bioremediation approaches have also relied on the injection of oxygen or oxygen-containing chemicals into the aquifer to provide electron acceptors to enhance aerobic biodegradation processes, however, this approach is not applicable to chlorinated solvents and other oxidized compounds.
In many aquifers, the cleanup rate is controlled by the rate of contaminant dissolution and transport by the mobile groundwater. When dense non-aqueous phase liquids such as halogenated aliphatic organic solvents are present or contaminants are present in lower permeable zones, dissolution rates are slow and a long time is required for aquifer cleanup. Under these conditions high operation and maintenance costs are a major problem.
Impermeable barriers are used to restrict the movement of contaminant plumes in ground water. Such barriers are typically constructed of highly impermeable emplacements of materials such as grouts, slurries, or sheet pilings to form a subsurface wall. When successful, these barriers eliminate the possibility that a contaminant plume can move toward and endanger sensitive receptors such as drinking water wells or discharge into surface waters. However contaminated groundwater often bypasses around these barriers unless they are constructed to completely enclose the contamination source.
Technologies to improve the chances that contaminated groundwater will encounter subsurface reactive agents have been developed. One such technique is the permeable reactive barrier (PRB), which is a passive in situ treatment zone of reactive material that degrades or immobilizes contaminants as groundwater flows though it. In contrast to subsurface walls, permeable reactive barrier walls do not constrain plume migration, but act as preferential conduits for contaminated groundwater flow. In a PRB, reactive materials are placed where a contaminant plume must move through it as it flows, with treated water exiting on the other side.
PRBs are installed as permanent or semi-permanent replaceable units across the flow path of a contaminant plume. Natural gradients transport contaminants through strategically placed treatment media. The media degrade, sorb, precipitate or remove chlorinated solvents, metals, radionuclides, and other pollutants. These barriers may contain reactants for degrading volatile organics, chelators for immobilizing metals, nutrients and oxygen to enhance bioremediation, or other agents.
The choice of reactive media for PRBs is based on the specific organic or inorganic contaminants to be remediated. Most PRBs installed to date use zero-valent iron (Fe0) as the reactive media for converting contaminants to non-toxic or immobile species. For example, Fe0 (can reductively dehalogenate hydrocarbons, such as by converting TCE to ethene, and can reductively precipitate anions and oxyanions, such as by converting soluble Cr+6 oxides to insoluble Cr+3 hydroxides. These barriers consist of a long trench constructed perpendicular to the groundwater flow that is backfilled with ground-up iron. As the chlorinated solvent and other contaminants flow through the barrier, they react with the iron and are transformed. The transformation reactions that take place in the barriers are dependent on parameters such as pH, oxidation/reduction potential, concentrations of the substrate(s) and contaminant(s) and reaction kinetics within the barrier. The hydrogeologic setting at the site is also critical, because geologic materials must be relatively conductive and a relatively shallow aquitard must be present to contain the system. The technology works well but is very expensive to construct. Examples include the work of Gillham et al. (1995, unpublished Communication to the International Containment Technology Workshop, Permeable Barriers Session, Baltimore, Md.). The disclosures of all patents and publications referred to herein are incorporated herein by reference.
Most PRBs are installed in one of two basic configurations: funnel-and-gate or continuous trench, although other techniques using hydrofracturing and driving mandrels are also used. The funnel-and-gate system employs impermeable walls to direct the contaminant plume through a gate, or treatment zone, containing the reactive media. A continuous trench may also be installed across the entire path of the plume and is filled with reactive media.
Pump-and-treat technologies and funnel and gate barriers are not conducive to broad site cleanup. These are interceptor technologies; there are no cost-effective technologies that address the entirety of the plume in situ.
Remediation techniques that have been employed for various contaminants are discussed more specifically below. Enhanced anaerobic bioremediation through reductive dehalogenation of halogenated aliphatic organic and inorganic compounds has been demonstrated as a method for remediating aquifers contaminated with chlorinated solvents (Holliger, 1995. Current Opinion in Biotechnol. 6:347-51; Beeman et al., 1994. In Bioremediation of Chlorinated and Polycyclic Aromatic Hydrocarbon Compounds, ed. Hinchee, et al., S K Ong, p. 14-27. Boca Raton: Lewis Publishers Ellis et al., 2000. Environmental Science and Technology. 34: 2254-2260). In this process an organic substrate is emplaced into the aquifer to stimulate the growth of anaerobic dechlorinating bacteria by providing an electron donor for energy generation and carbon source for cell growth (Lee et al., 1997. J. Ind. Microbiol. Biotechnol. 18(2/3):106-15; McCarty et al., 1994. Handbook of Bioremediation, Lewis Pub., Boca Raton, Fla., pp. 87-116). For example, tetrachloroethene (PCE) and trichloroethene (TCE) can be treated by the following reaction:
PCE- greater than TCE- greater than cis DCE  greater than VC- greater than ethene
Cis-dichloroethene (cis-DCE) and vinyl chloride (VC) are produced as intermediate compounds by this reaction. However, when a suitable microbial population is present, cis-DCE and VC are completely degraded to the non-toxic end product ethene.
Perchlorate can be biodegraded to chloride under anaerobic conditions through the sequence:
ClO4xe2x88x92(perchlorate)xe2x86x92ClO3xe2x88x92(chlorate)xe2x86x92ClO2xe2x88x92(chlorite)xe2x86x92Clxe2x88x92(chloride)
This process requires the addition of an organic substrate to remove dissolved oxygen, which can inhibit this process, and provide reducing equivalents to drive the reaction. (Herman et al., 1998. Journal of Environmental Quality, 27: 750-754). Studies on perchlorate degradation are primarily laboratory scale. Full-scale applications have been limited to treatment of wastewaters generated from handling rocket propellants in industrial situations.
A variety of inorganic compounds including chromium (Cr), uranium (U) and technetium (Tc) are more mobile in subsurface environments in a more oxidized state. By promoting anaerobic, reducing conditions, these compounds can be converted to a more reduced, less mobile state that will promote their immobilization. For example, chromium commonly occurs in two oxidation states in the environment: Cr[III] and Cr[VI]. The oxidized form, Cr[VI], is relatively mobile in the subsurface existing in solution as the HCrO4xe2x88x92 and CrO4xe2x88x922 ions. The reduced form, Cr[III], is essentially immobile in ground water. Cr[III] may be removed from solution as an amorphous precipitate (Cr(OH)3) or as a solid solution with other metal oxides and hydroxides (Fe(OH)3) (Palmer et al., 1994, Natural Attenuation of Chromium in Groundwater and Soils, EPA Ground Water Issue, EPA/540/5-94/505). Studies on reductive immobilization of heavy metals and radionuclides are primarily laboratory scale.
The patent of Suthersan (U.S. Pat. No. 5,554,290) utilizes an in situ anaerobic reactive zone for in situ precipitation and filtering out of dissolved heavy metals as metallic sulfides, and microbial denitrification to degrade nitrate to nitrogen gas. Although dithionite has also been injected into wells to react with contaminants and precipitate in place, use of dithionite is less attractive due to its toxicity and cost.
Examples of bioremediation using soluble substrates include the accelerated anaerobic pilot test (AAPT) conducted by the Remediation Technologies Development Forum (RTDF), the hydrogen releasing compound (HRC(copyright)) and work with molasses. The AAPT evaluated the effectiveness of injecting lactate dissolved in water into the aquifer for establishing the reducing conditions necessary for the reductive dechlorination of TCE and cis-DCE to ethene. The treatment was performed using a closed-loop approach, which included three up-gradient injection wells and three down-gradient recovery wells. Recovered groundwater was amended with lactate and re-injected into the up-gradient wells, thus closing the loop. Lactate is a soluble readily biodegradable substrate. The results of this study were that lactate could effectively promote anaerobic dehalogenation of the chlorinated solvents to non-toxic end products, but lactate addition resulted in biofouling of subsurface equipment.
HRC(copyright) is a commercially available lactate-based polymer material with a glycerol coating formulated and sold by Regenesis, Inc. (San Clemente, Calif.). It is reported to offer long-term availability of lactate (electron donor) to the aquifer via a time-release mechanism. In the subsurface, HRC(copyright) slowly hydrolyzes, releasing dissolved lactate that travels out into the aquifer enhancing reductive dehalogenation.
Molasses has been used for bioremediation studies because of its ready availability, inexpensive cost. and rapid biodegradability. When molasses was introduced into the aquifer as an electron donor via an infiltration gallery that was dug to a depth immediately above the shallow groundwater table at a site in Lumberton, N.C., some biofouling was evidenced within one month of startup.
An early description of the use of insoluble oils in reductive dehalogenation is by Dybas et al. (1997, In Situ and On Site Bioremediation 3.59, Papers from the 4th Int. In Situ and On Site Bioremediation Symp., New Orleans, La.). Examples of bioremediation using insoluble substrates include work with soybean oil by Parsons Engineering Science (PES) (Denver, Colo.) and at an industrial site in Hamilton, N.C. Work by PES at Defense Depot Hill Utah, DDHU and at the Department of Energy Facility (DOE, Pinnellas, Fla.) employs the direct injection of soybean oil in a field demonstration. In each study, one injection well was injected with excess soybean oil. The effects of the introduction of oil were monitored in a set of down-gradient monitor wells. Results in the two studies indicate the initial absorption of the chlorinated solvents into the oil, followed by slow dissolution of the solvents back into the groundwater and their subsequent reductive dechlorination. At the Hamilton, N.C. site, a full-scale oil injection was performed by Solutions Industrial and Environmental Services, Inc. (Raleigh, N.C.), with approximately 200 injection points that were located throughout the chlorinated solvent plume. Each injection point was injected with liquid soybean oil and the temporary injection well was removed.
The patent of Frederickson et al. (U.S. Pat. No. 5,265,674) discloses treatment of aquifers using an oil, such as vegetable oil or mineral oil, which is chosen to be less dense than water, so that the oil rises through the water and contaminant plume. In this method, reliance is placed on partitioning of the contaminant in, and rising with, the rising oil. In this work, mineral oil was preferred because of its slower biodegradation rate.
It is an object of the invention to provide a safe, low-cost, effective method of bioremediation of aquifers using emulsified oil in the form of an oil microemulsion. The method of the invention enhances a wide variety of anaerobic biodegradation processes in the subsurface by providing a biodegradable, immobile organic substrate. Emulsified food-grade insoluble oil is an inexpensive electron donor source. In the aquifer, the emulsion of the invention can provide for a naturally coupled metabolic reaction between oil-degrading microorganisms and dehalorespiring microorganisms. Using emulsified oil according to the invention allows for improved distribution of the oil laterally away from the injection points and entrainment of the oil micro-droplets into the effective pore space of the aquifer material. In addition, the method of the invention may be implemented in a variety of configurations, including PRB and broad area coverage.
Use of emulsified oil for in situ degradation of halogenated organic compounds and perchlorate and for reductive immobilization of other contaminants is a one-time activity. The naturally slow rate of substrate dissolution and biodegradation establishes a naturally occurring time-release mechanism so that only the amount of substrate is used that will result in the desired biodegradation. Little substrate is xe2x80x9cwastedxe2x80x9d by non-specific biodegradation processes. The improved method of distribution allows the process to be implemented in a variety of configurations including PRB and broad area coverage. The use of vertical injection wells offers the advantage of being able to place the oil emulsion in desired strata, or throughout the entire depth as desired.
Other objects and advantages will be more fully apparent from the following disclosure and appended claims.
The invention herein is a method for remediating aquifers and groundwater contaminated, for example, by toxic halogenated organic compounds, certain halogenated inorganic compounds, and oxidized heavy metals and radionuclides, using the introduction of an innocuous oil, preferably an edible, food-grade oil, preferably formulated into a microemulsion by mixing with one or more natural food-grade emulsifiers (such as lecithin) and water. The invention provides a specific, time-release method of bioremediation. Pretreatment of the aquifer increases mobility of the emulsion through the aquifer.